1. Field of the Invention
The present invention relates to a correction method for synchronization with ISDN (Integrated Service Digital Network) in cell station system for use in a private-network-use PHS (personal handy-phone system) and a circuit therefor, in premises for example, and more particularly to a correction method for synchronization with ISDN. capable of preventing missing out of a personal station when the cell station is connected to the ISDN and a circuit therefor.
2. Description of Related Art
In the private-network-use PHS, its cell station is connected to the ISDN depending on the case. When it is not connected to the ISDN, the cell station is required to itself send a super frame which is a signal for capturing a personal station. Further, upon connecting the cell station to the ISDN, the cell station system is required to extract a clock from a network or the ISDN and obtain synchronization between the clock of the network side and a clock of the air side so as to transmit the super frame at a timing of the network side. The air side is a radio section between the PHS cell station and PHS personal station.
For a conversation through the ISDN in premises, generally, the speech path is connected to the ISDN only during a conversation, and after the conversation ends, connection with the ISDN is canceled. That is, after the conversation ends, no network clock is extracted from the ISDN. Therefore, synchronization with the network is lost and if a conversation is intended again, it is necessary to attain synchronization with the network again.
In a conventional PHS cell station system, a phase comparator circuit and a dividing/synchronization correction circuit are connected in series so as to generate a clock for air. The phase comparator circuit receives the clock for air to be outputted to the air side and a clock for the network side for phase comparison and compares them so as to output a phase difference signal. And a dividing/synchronization correction circuit corrects the clock synchronization for air generated by dividing the master clock in accordance with that phase difference signal.
In such a structure, the clock for air and the clock for the network side are connected directly to the input. Therefore, upon connecting the cell station to the ISDN, at an instant when it is connected to the ISDN, synchronization with the network is carried out. However, if synchronization with the network side is carried out at the instant when it is connected to the ISDN, in the cell station, the phase of a clock for air on current use for communication with the air may deviate. In such a case as to deviate, the cell station is missed out a personal station with which is communicating through the air, thereby communication between the cell station and personal station being broken.
A public use cell station has solved this problem by using a phase locked loop (PLL) circuit having a longer time constant. However, because, in the private-network-use communication apparatus, setting of connection with the ISDN and cancellation of the connection therewith are frequently repeated, a function for enabling a start of communication immediately after connection is established is set therein. Use of the PLL circuit having a longer time constant is not suitable for conditions of the private-network-use PHS.
Accordingly, it is an object of the present invention to provide a correction method for synchronization with ISDN in cell station system for use in private-network-use PHS, capable of solving the above described problems and a circuit therefor.
According to the correction method for use in the private-network-use PHS of the present invention, after synchronization between air side and between ISDN side is assured based on a master clock, the clock synchronization is commenced a correction depending on a phase difference detected by comparing phases thereof.
Thus, a correction circuit according to the present invention comprises: a permission signal generator circuit, a cancellation signal generator circuit, a divider circuit for air, a divider circuit for synchronization, a network side divider circuit, and a phase comparator circuit.
The permission signal generator circuit generates a correction permission signal for network synchronization by connection with ISDN. The cancellation signal generator circuit outputs a reset cancellation signal for network synchronization synchronously with a master clock in the system if a clock is extracted from ISDN after it receives the correction permission signal from the permission signal generator circuit. The divider circuit for air operates regardless of a connection state with the ISDN for dividing the master clock of the system to output a clock for air, the outputted dividing ratio being changed depending on a phase difference signal to be generated elsewhere. The divider circuit for synchronization divides the master clock to a predetermined frequency by receiving a reset cancellation signal from the cancellation signal generator circuit. The network side divider circuit divides a clock extracted from the ISDN to the predetermined frequency by receiving a reset cancellation signal from the cancellation signal generator circuit. And a phase comparator circuit detects a phase difference between an outputted clock of the divider circuit for synchronization and an outputted clock of the network side divider circuit when the correction permission signal is outputted from the permission signal generator circuit so as to output the phase difference signal.
With such a circuit structure, when the cell station is connected to the ISDN and clock synchronization correction is permitted, the divider circuit for synchronization and network side divider circuit respectively for generating a clock for phase comparison for synchronization correction can start their dividing operation at the same timing. That is, in case of being frequently connected to the ISDN for each conversation like the cell station for use in the private-network-use PHS, clock synchronization correction is started in a condition in which synchronization between the air side and ISDN side is assured. Thus, there never occurs a happening in which air side clock is disturbed when the cell station is connected to the ISDN. Therefore, the cell station never misses out its personal station during a communication therewith.